Important

The ADD action only supports Number and set data types. In general, DynamoDB
recommends using SET rather than ADD.

Use the ADD action in an update expression to do either of the following:

If the attribute does not already exist, add the new attribute and its
value(s) to the item.

If the attribute already exists, then the behavior of ADD depends on the
attribute's data type:

If the attribute is a number, and the value you are adding is also a number,
then the value is mathematically added to the existing attribute. (If the
value is a negative number, then it is subtracted from the existing
attribute.)

If the attribute is a set, and the value you are adding is also a set, then
the value is appended to the existing set.

This object is as immutable (or unmodifiable) as the underlying value (of
type UnitOfExpression) given during construction.